Inductance coil



Dec. 20, 1932.

P. F. SCOFIELD INDUCTANCE 0011..

Filed Feb. 24, 19 31 INVENTOR, PHIL [P F SCOFIEL D.

ATTORNEY v Patented Dec. 20, 1932 UNITED STATES PATENT" OFFICE 2mm 1?. scormr-n'or ram limo; cnmronma, assrenoa 'ro rmmrz a moi-nan, mm. or sun raaucrsco, camroanm, a oonroaarron or nnvana INDUCTANOE COIL Application filed February 24, 1931. Serial No. 517,851.

fixed regardless of variation in temperature or other workin conditions.

Among the o jects of my invention are:

First, to provide an inductance whose variation with temperature is practically nil; sec-' ond, to provide a coil having a relatively low resistance at radio frequencies; third, to provide a coil which may be cheaply and easily constructed from readily obtainable materials; and fourth, to provide a coil which is ri 'd and self-supporting.

Ot er objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the sco e of the claims;

eferringto the drawing:

Figure 1 is an elevation and Figure 2 is a plan iew of an inductance coil made in accordance with my invention, ortions of Figure 1 being cut away in the rawing toshow the construction.

Fi re3 is a cross-section used 1n the coil.

The inductance of an conductor depends both upon the length 0 I the conductor, upon its cross-section, and particularly upon its dis osition relative to other conductors with which it is magnetically linked. The usefulness of an inductor in an oscillating circuit also demandsthat ithave low resistance. It is further desirable that it be compact, not only for purely mechanical reasons, but also to reduce accidental linkages with other circuits. For these reasons it is customary to form inductors as helices or spirals, so that each element of the conductor may be linked magnetically with as many other elements as possible, thus reducing the length and therefore the resistance of conductor required to give a given inductance.

All of the materials having high conducof the conductor tivity have also relatively large coeficients of thermal expansion. A rise in tem erature of the coil therefore increases its in uctance, not merel in direct roportion to its increase in ength, but also by increasing the flux linkages with the other turns of the coil.

Where the inductor is made a portion of an oscillating circuit which must be operated as nearly as possible at a standard frequency, or one which is used to measure frequency, as m a wave meter, it is necessary that such coils as those described be operated under carefully predetermined thermal conditions, in order that errors due to temperature variations shall not be introduced. In cases where temperature control boxes will not give the required results, i. e., in inductances. for radio transmitters of such power that amaterial amount ofheat is generated" in the coils themselves, such devices as water-cooling have been introduced to maintain the temperature substantiall constant. 1

Broadly consi cred, this invention co prises an inductance coil wherein the conductor is formed of a core which has a substantiall zero coefiicient of linear e ansion, surroun ed by a shell or jacket of h! h conductive material such as copper, whic material may also have a relatively high coeflicient of thermal expansion. Such a conductor, when used in a radio frequency circuit, carries substantially all of the current in its highl conducting shell, and its resistance is there ore not measurabldifferent from that of a solid conductor. oreover, the conductor has two coeflicients of expansion; a linear coefiicient of expansion which is but slightly greater than that of the core, and a diametral coefiicient of expansion which is markedly greater. .Since the inductance-of the conductor decreases with increase in diameter, these two coefiicients of expansion serve to neutralize each other, so that the thermal eoeflicient of inductance of the completed coil ma be made substantially zero.

he particular form of inductor shown in the drawing is a self-supporting coil of a type applicable to transmitting circuits. It will be understood that the same principle may be applied to coils of fine wire wound on forms, such as are frequently used in wave meters and other low power circuits, the difference merely being one of dimension and not of principle.

Describing in detail the coil shown in the drawing, it comprises a plurality of turns 5 of wire or rod comprising a core 6 (see Figure 3) which is preferably of the nickel steel alloy which is known commercially as invar and which is composed of approximate- 1y 36% of nickel and 64% of iron. Such an alloy may be obtained having a coeflicient'of thermal ex ansion which is about .8X 1O- per degree and which may even be negative over a limited temperature range in selected samples.

Surrounding this core and welded or brazed thereto is a shell 7 which is preferably of co per or other highly conductive material. is shell or jacket has a relatively high coefiicient of expansion, but since the core has a much greater mechanical stren th than the shell, and since the shell is fixe rigidly to the core, the resulting conductor has a linear coeflicient which is extremely small, the hi hly expansive copper being constrained by the more rigid steel core.

The volumetric coefficient of expansion of the copper remains, however, unchanged, and the composite conductor retains an appreciable diametral coeflicient of expansion, which is greater than the linear expansion of that portion of the conductor which is formed of co r would be were the copper unstressed.

he coil is held by a pair of clamps 8, preferably of fused uartz, which is also a material having a neg igible coefiicient of expansion. The quartz clam s may be retained in position by bolts 9. similar pair of clamps 11 is shown as positioning the terminals of the coil.

In practice, the extremely slight linear coeflicient of expansion, which tends to increase the inductance of the coil, is ofi'set by the diametral coefiicient which decreases the inductance. Moreover, although the core ma terial is of high resistance and is magnetic, its net efiect at high frequencies is sensibly zero, and experience has shown that these inductance coils operate over wide temperature ranges even more accurately than watercooled coils, and practically as accurately as light duty coils operated under thermal control.

I am aware that composite wires of the t described have been used as lead-ins in e ectric lamps and elsewhere, as a substitute for platinum. In these cases, however, the effort has been to give the compound wire a diametral coefiicient of expansion equal to glass, and the difierence between the diametral and longitudinal coefiicients of expansion has been ignored.

In the present invention it is the negligible linear coeflicient of expansion which is the dominant factor, the diametral coeflicient serving merely to give a second order correction. i

A modified form of my invention which is particularly useful in circuits operating at very high frequencies, utilizes a core 6 formed by a fused quartz rod formed into a helix, and having a conductive shell lated thereon. In appearance this form 0 coil may be identical with that already described, the diflerence residing in the materials and (H mensions only. The shell 7 may be of copper, but for this service I prefer to use silver, depositing the initial coat by one of the well known processes which are used for forming mirrors.

Thus, the quartz helix may be introduced into a warm solution of silver nitrate and ammonia, and a reducing agent such as Rochelle salt, tartaric acid, or glucose added. This forms a firml adherent coat of silver, upon which as thick a layer of silver or copper as is desired may be clectro lated.

Since the skin effect at high requencies is very great, a relatively thin shell is ample to supply the greatest possible conductance, and owing to t e great rigidity of the quartz as compared to the metal coating the longitudinal expansion of the conductor is practically that of the fused quartz; i. e., about 26X 10' er degree (1., as compared to about 17 X10 or copper or silver. A relatively thin shell of metal will ive the diametral expansion to compensate or this.

Other low expansion cores, such as Pyrex or N onex glass may be used where the conditions are less rigorous.

I claim:

1. An inductor comprising a conductor disosed in a coil of at least one turn, said conuctor havin a core of material having a low coeflicient 0 thermal expansion fixed to a shell of conducting material having a high electrical conductivity and a relatively high c'oeflicient of expansion.

2. An inductance coil comprisin a conductor having a core of nickel stee of negligible coefliclent of expansion and a copper shell fixed to said core.

3. An inductor comprising a conductor disposed in a coil of at least one turn and having a plurality of layers, said conductor having a combined diametral coefiicient of thermal expansion which is relatively great as com ared to the combined longitudinal coefliclent of thermal expansion thereof.

4. In an inductance coil, a conductor having a'relatively low longitudinal coeflicient of expansion and a relativel high diametral coeflicient of expansion, and means for main taining said coil in its predetermined form, said means comprising a material whose dimensions are substantially unaffected by temperature.

5. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor having a core of relativel low coefficient of expansion and high rigidity, and a conductin shell fixed to said core and constrained t ereby against expansion in the direction of the length of the core.

6. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor having a core of nonconducting material having a low coefiicient of thermal expansion fixed to a shell of conducting material having a high electrical conductivity and a relatively high coeflicient of thermal expansion.

7. An inductance coil characterized by having a substantially constant inductance under varying temperatures, comprising at least one turn of a quartz rod plated with metal of high conductwity and having a high coeflicient of thermal expansion.

8. The method of maintaining the inductance of a coiled conductor havin a shell of material of a. high coeflicient 0 thermal expansion constant under varying temperatures, which comprises restricting the lon tudinal expansion of said shell without 1nterferin with the diametrical expansion of said shefi.

In testimony whereof, I have hereunto set my hand.

PHILIP F. SCOFIELD.

DISCLAIMER 1,891,481.Phil11p F. Scofield, Palo Alto, Calif. INDUCTANOE COIL. Patent dated December 20, 1932. Disclaimer filed December 2, 1935, by the patentee. Hereb enters this disclaimer to claims 1, 5, and 6 of said Letters Patent which are in the ollowing words, to wit:

"1. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor havm a core of material having a low, coefiicient of thermal expan- S1011 fixed to a shell 0, conducting material having a high electrical conductivity and a relatively coeflicient of expansion. i

"5. An inductor comprising a conductor dis ed in a coil of at least one turn, said conductonhavmg a core of relatively low coe cient of e ansion and high rigidity, and a conducting shell fixed to said core and constrained t ereby against expansion in the direction of. the length of the core.

"6. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor having a core of nonconducting material having a low coefficient of thermal expansion fixed to a shell of conduc material having a high electrical conduct1v 1t and a relatively high coeificient of rmal expansion.

[ Gazette January 14, 1936.]

posed in a coil of at least one turn, said conductor having a core of relativel low coefficient of expansion and high rigidity, and a conductin shell fixed to said core and constrained t ereby against expansion in the direction of the length of the core.

6. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor having a core of nonconducting material having a low coefiicient of thermal expansion fixed to a shell of conducting material having a high electrical conductivity and a relatively high coeflicient of thermal expansion.

7. An inductance coil characterized by having a substantially constant inductance under varying temperatures, comprising at least one turn of a quartz rod plated with metal of high conductwity and having a high coeflicient of thermal expansion.

8. The method of maintaining the inductance of a coiled conductor havin a shell of material of a. high coeflicient 0 thermal expansion constant under varying temperatures, which comprises restricting the lon tudinal expansion of said shell without 1nterferin with the diametrical expansion of said shefi.

In testimony whereof, I have hereunto set my hand.

PHILIP F. SCOFIELD.

DISCLAIMER 1,891,481.Phil11p F. Scofield, Palo Alto, Calif. INDUCTANOE COIL. Patent dated December 20, 1932. Disclaimer filed December 2, 1935, by the patentee. Hereb enters this disclaimer to claims 1, 5, and 6 of said Letters Patent which are in the ollowing words, to wit:

"1. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor havm a core of material having a low, coefiicient of thermal expan- S1011 fixed to a shell 0, conducting material having a high electrical conductivity and a relatively coeflicient of expansion. i

"5. An inductor comprising a conductor dis ed in a coil of at least one turn, said conductonhavmg a core of relatively low coe cient of e ansion and high rigidity, and a conducting shell fixed to said core and constrained t ereby against expansion in the direction of. the length of the core.

"6. An inductor comprising a conductor disposed in a coil of at least one turn, said conductor having a core of nonconducting material having a low coefficient of thermal expansion fixed to a shell of conduc material having a high electrical conduct1v 1t and a relatively high coeificient of rmal expansion.

[ Gazette January 14, 1936.] 

